When handling explosive materials and products, and in environments where highly inflammable gases or materials are present, the formation of sparks poses a potential risk for explosion and/or fire. For example, the air in certain industrial environments, particularly in the field of chemical industry, may comprise gasses generated by volatile substances in dyers baths, solvents, cleaning compounds, adhesive substances, etc., which can easily be ignited by the slightest spark, which of course may result in disastrous consequences. Dust stuff that can easily catch fire may also occur. In such environments, it is therefore important to take measures in order to eliminate such risks.
A spark can easily appear by the discharge of static electricity or when hard objects hit each other, e.g. metallic tools, instruments or machines hitting something or being dropped to the floor. The formation of sparks during shocks can be avoided by covering the objects with a softer material, such as plastics or rubber, which is not always a good solution in practice, however. In the case of static electricity, it is a problem that more or less electrically conductive objects are electrostatically charged during handling, in order to finally be discharged at the formation of a spark against another object, or a person, e.g. being in contact with earth.
Such charging of objects can be prevented by having the present object in constant connection with earth for continuous drainage of generated static electricity. For example, a metallic object such as a tool, a sheet-metal bench or a machine casing, may be connected to earth by applying a conductive clamp or the like to the object, wherein the clamp is connected to earth by means of a cable or the like.
In FIG. 1, a commonly occurring grounding clamp 100 made in a conductive metal is shown, comprising two pointed opposite contacting means 102a and 102b, which are held pressed against each other by means of a spring 104, for contacting the object. The contacting means can be opened by hand by means of two hingedly connected arms 106a and 106b, each being connected to a contacting means 102a, b, respectively. At least one of the arms 106a is by means of a cable 108 further connected to a point of dissipation P, such as earth. Once the metal clamp 100 is applied to an object, the contacting means 102a, b will dissipate current from the object through the cable to earth, such that it cannot be recharged electrostatically.
Other types of grounding clamps are also occurring, e.g., C-shaped clamps with screwed contacting means, and pipe clamps with two semi-circular contacting means which are screwed together towards each other around a pipe or a rod to be drained. The contacting means occurring in these grounding clamps are typically provided with at least one metal point or spike, in order to provide a safe electric contact to the object to be drained, such that it can penetrate dirt, layers of oxide, paint coats or the like that may often occur.
However, a problem with such previously known clamps is that an object has already been charged to some extent when the clamp is to be connected thereto. A spark may thereby be released unintentionally as the clamp instantaneously discharges the object when initially contacting it, or possibly when the cable 108 is connected to earth after the clamp has been applied at the object, possibly resulting in an explosion or fire. Such a discharge may also be caused by a capacitance being built up in the clamp itself, especially if its metallic mass is relatively great. Furthermore, there is a certain risk that the clamp is dropped to the floor or bumps into some object during handling thereof, such that a spark may even then be released, a so-called slag spark.
SU 856048 A discloses a discharging device for high voltage applications, which is used to remove a charge from a conductor where electrical energy has been accumulated. When applying the device to the conductor, a spark appears with a current limited by a semi-conductor rod in the device.